In cellular networks, where the communication connections are separated from each other using code division multiple access (CDMA) technique, a mobile station having an active communication connection with the cellular network should be able to receive data at the radio frequency related to that communication connection practically all the time. In an interfrequency handover the frequency on which an active communication connection exists is changed. A cell change may accompany the interfrequency handover, in which case the maneuver is an intercell-interfrequency handover, or the frequency change may take place within a single cell meaning that an intracell-interfrequency handover is performed. The present invention is equally applicable to all interfrequency handover types. Furthermore, the present invention is also applicable to intersystem handovers, where a handover is made from a first cellular system employing CDMA to a second cellular system employing TDMA (Time Division Multiple Access). A handover from a Wideband CDMA system to GSM (Global System for Mobile Communications) is an example of such handover.
In order to find a suitable target frequency for an interfrequency handover the mobile station must evaluate the available target frequencies in terms of connection quality that it could achieve on them. This in turn necessitates that the mobile station must quickly tune its radio receiver (or one of its radio receivers, in case it comprises several of them) onto each target frequency to be evaluated for a certain period of time. In TDMA systems this is not a problem since the mobile terminal must anyway transmit and receive only during certain typically cyclically occurring time intervals, between which it has time to tune its receiver onto whatever other frequencies it wants. However, in other systems like CDMA (Code Division Multiple Access) where reception and transmission are substantially continuous, it may be problematic to find suitable time intervals for the measurements.
Typically data is transmitted over the radio interface in frames, which have a certain number of time slots. The time slots comprise a certain number of symbols. The number of time slots in a frame, the number of symbols in a time slot and the duration of a symbol are usually defined in the applicable cellular system specifications.
It is known to define and employ a slotted mode (or compressed mode) for transmission and reception in order to leave certain time intervals free for measurement purposes. Slotted mode means that both transmission and reception are performed only according to a certain predefined slot pattern. FIG. 1 illustrates a set of frame chains of which frame chain 101 corresponds to uplink transmission in normal mode, frame chain 102 corresponds to downlink transmission in normal mode, frame chain 103 corresponds to uplink transmission in slotted mode and frame chain 104 corresponds to downlink transmission in slotted mode. The relative lengths of the slotted frames and the silent periods between them are defined in the applicable system specifications. The frames in the slotted mode may carry the same amount of user data as frames in the continuous mode or less user data than frames in the continuous mode. Typically the uplink and downlink silent periods are at the same time. FIG. 1 shows an example, where the silent periods in the uplink and downlink direction occur at different times.
In a single-receiver station slotted receiving is considerable in order to reserve the receiver to the use of the ongoing connection for only a part of the time. Slotted transmitting is not that considerable at first sight, but usually it can be required or even it can be unavoidable since the transmitter should be substantially powered down for those time periods when the receiver is measuring.
An interfrequency handover typically improves the quality of a communication connection, when the handover is made to a target frequency, for which there is not present a radio transmitter (source of interference) at an adjacent frequency. As preparatory measurements relating to a handover at available target frequencies typically require slotted mode, they may cause interference to other communication connections employing the own frequency. A method, where a need for measurements relating to an interfrequency handover is detected reliably, is therefore needed.
Need for an interfrequency handover may be detected by determining the quality of the current communication connection or by estimating adjacent channel interference, for example, interference caused by a radio transmission at an adjacent frequency, at the own frequency. It is possible to determine the quality of the current communication connection or to estimate adjacent channel interference from measurement in the own frequency in the continuous mode. The estimation of adjacent channel interference is described, for example, in a patent publication WO 01/31958 A1. Such measurements at the own frequency do not cause interference to other communication connections employing the same frequency. Once the quality of the communication connection is determined or the adjacent channel interference is estimated, it is typically compared to a predetermined threshold value to determine, whether there is need to initiate preparatory measurements at available target frequencies. A proper threshold value for initiating preparatory measurements may vary, and it is not necessarily easy or straightforward to determine suitable threshold values.